There have been a number of different valve assemblies designed to prevent the fluid level in tanks from exceeding a predetermined level of fluid therein. In particular, with a tank used to contain compressed or liquified gas, such as propane, it is important that the liquified gas loaded into the tank under pressure not exceed a certain fluid level in the tank so that a sufficient amount of head space at the top of the tank is maintained for safety purposes. Because of the high coefficient of thermal expansion of liquified propane gas, increases in ambient temperatures can cause the head space in the tank to be rapidly consumed. The tanks are rated so as to withstand a predetermined amount of internal pressure; however, if the tank is overfilled beyond the maximum level, there is the danger that the internal pressure may exceed the rated strength of the tank, particularly if exposed to excessive temperatures, and lead to potentially dangerous failures of the tank walls, and/or lead to dangerous expulsion of gas or liquid from a relief valve.
In this regard, it is known to provide the tanks with a dip tube that extends through the tank wall into the tank interior space to a specified level below the maximum fill level with its other end open to ambient so that once the fluid level reaches the bottom end of the tube, the liquified gas begins to be evacuated from the tank. As is apparent, if the operator who is filling the tank does not detect this condition and continues to fill the tank, a potentially hazardous situation around the tank filling station is created.
As mentioned, there have been a number of prior art devices that prevent further filling of the tank once the maximum fill level is attained. One of the problems with some of these prior valve devices is that they are not readily adapted for use with current service valves that are already in place on the tanks. In this regard, many of these valves do not allow for outgoing flow therethrough or, if they do so, only allow such reverse flow in relatively small amounts. Another shortcoming with a majority of these devices lies in their complexity, as many require a large number of different parts. For example, in a number of prior art patents, the valves utilize a primary valve member and a secondary valve member controlled by a float assembly. The primary valve blocks incoming fluid flow when the float assembly is operable at maximum fill conditions to seat the secondary valve. When the secondary valve seals its associated port, there is a fluid pressure buildup between the secondary and primary valves that pushes the primary valve onto its seat sealing lateral ports to stop fluid flow into the tank. The use of two moving valve members, and the large number of associated parts in such a valve assembly, is undesirable in terms of driving up the cost for the valve, increased time and complexity for assembly thereof, the greater likelihood of its failure under repeated cycling over time, and the ability to provide for adequate outgoing flow therethrough.
Another problem with prior valve devices is that they have trouble providing precise and repeatable fluid flow cutoff during tank loading operations once the maximum fill level in the tank is obtained. Pertinent regulations specify that a predetermined amount of head space remain above the maximum fluid level for safety purposes, as mentioned. However, with prior valve devices, the point at which they cut-off fluid intake into the tank during loading operations can vary in terms of the fluid level in the tank from the predetermined maximum fill level by as much as 11/2 percent.
Accordingly, there is a need for a simpler and less expensive valve assembly that prevents overfilling of tanks beyond a predetermined fluid level therein. More particularly, it would be desirable to provide an overflow protection valve assembly that can be attached to a service valve such as on an LP tank for allowing flow into and out from the tank through the valve assembly. A valve assembly that provides more precise fluid flow cut-off at the predetermined maximum fill level would also be desirable.